1. Field of the Invention
This invention relates to an acoustic resonator for attenuating sound in a conduit.
2. Background of the Invention
Mechanical air control equipment for a Heating Ventilation and Air Conditioning (HVAC) system can be a major source of sound in a building. If the sound generated by the mechanical equipment is obtrusively loud its effect can have serious consequences on the overall environment in a building. Air distribution ducting in an HVAC system can act as a transmission path for the unwanted sound throughout a building. Additionally, fluid flowing through abrupt changes in the cross-sectional dimensions of a duct can also produce sound. The sound created by a mechanical device or within the ducting system can travel upstream in a return air duct and downstream in a supply air duct and thus be heard by an occupant of a room within the building. Various sound sources within the duct include, but not are limited to, circulating fans, grills, registers, diffusers, air flow regulating devices, etc. Accordingly, there has been a longstanding problem with the amount of sound which is transmitted through the ducting of an HVAC system.
Various attempts have been made to minimize the sound in air ducting. One such system, commonly referred to as a dissipative silencer, provides a sound attenuating liner either inside or outside the duct. The material may be foam, mineral wool or fiberglass insulation. These materials moderately attenuate sound over a broad range of frequencies; however, these liners are sometimes not desirable because of space requirements and the extended length of coverage required to produce adequate attenuation.
Additionally, reactive silencers have been used to attenuate sound. They typically consist of perforated metal facings that cover a plurality of tuned chambers. The outside physical appearance of reactive silencers is similar to that of dissipative silencers. Generally, reactive silencers attenuate low frequency sounds. Because broad band sound attenuation is more difficult to achieve with reactive silencers than with dissipative silencers, longer lengths may be required to achieve similar sound loss performance.
Another attempt to reduce the noise in a duct includes producing an inverse sound wave that cancels out unwanted noise at a given frequency. An input microphone typically measures the noise in a duct and converts it to an electrical signal. The signal is processed by a digital computer that generates a sound wave of equal amplitude and 180.degree. out of phase. This secondary noise source destructively interferes with the noise and cancels a significant portion of the unwanted sound. The performance of these active duct silencers is limited by, among other things, the presence of excessive turbulence in the airflow passage. Typically, manufacturers recommend using active silencers where duct velocities are less than a 1500 feet per minute (FPM) and where the duct configurations are conductive to smooth evenly distributed airflow. These operational parameters limit the broad usage of the canceling sound technique. Additionally, the high cost of a sound cancellation system further limits its use. The present invention addresses the limitation of the prior art and provides an acoustic resonator that attenuates the sound carried in the air control system.